Regulators, or converters, including a switch, sometimes referred to as a power switch, for transferring energy from an input, such as an AC or DC voltage or current source, to a regulated output are well known. In some regulators, sometimes referred to as switching regulators, the switch turns on and off to regulate the output. In other regulators, sometimes referred to as linear regulators, the switch operates in its active, or saturation region.
Common switching regulator configurations include Buck, Boost, Buck-Boost, flyback, SEPIC, Cúk, half bridge, and full bridge to name a few. As is also well known, various control methodologies for controlling conduction of the power switch can be applied to switching regulators, including Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM), and for each of these control methodologies, various feedback and feed forward techniques are possible including voltage mode control and current mode control.
Some switching regulators include an error amplifier which is responsive to a reference signal and to a feedback signal proportional to the regulator output to generate an error signal. The error signal is then used to generate the switch control signal. For example, in the case of voltage mode PWM control, the error signal is compared to a ramp signal to generate the switch control signal with a duty cycle suitable for regulating the output voltage to the desired level.
Many conventional switching regulators include a so-called “soft start” feature whereby the regulator output is ramped up slowly in order to avoid large and potentially damaging inrush current and/or voltage overshoot when power is applied to the regulator. One common way of achieving soft start is by slowly ramping up a reference voltage to the error amplifier, such as by slowly charging a capacitor through a resistor with the RC network having a time constant selected to ensure that inrush current is limited. By applying a slowly rising reference to the error amplifier, the regulator output increases slowly, and thus the current is reduced according to I=C·dv/dt due to the smaller dv/dt. The slow ramping of the reference voltage can also be achieved with a charge pump, digital-to-analog converter (DAC), or other methods known to those skilled in the art.
Dropout refers to a condition in which the regulator input falls to a level that is too low to allow for satisfactory output regulation. For example, dropout voltage is a term sometimes used to describe the minimum input voltage that is required to maintain output voltage regulation. Since recovery from a dropout condition requires the regulator to operate at maximum duty cycle following a period of operating with less than sufficient input, significant inrush current and/or voltage overshoot can occur.